This invention concerns fiber reinforcement of a flange for fiber reinforced pipe.
There is a substantial market for pipe made by winding glass fibers coated with a plastic material around a mandrel and curing the resin. Such fiber-reinforced plastic pipe is lightweight, has high burst strength, and by choice of suitable plastic resins, can be quite resistant to attack by the materials carried in the pipe or the environment in which the pipe is used. Various polyester resins, epoxy resins and the like may be used. The types of resins or fibers employed in practice of this invention are not material, and glass fiber, epoxy resin and the like mentioned herein are merely exemplary.
Glass fiber reinforced pipe sections may be secured end to end or to pipe fittings, valves, or the like, by bolts connected to flanges on the pipe. It is conventional to form pipe flanges separately from the reinforced pipe. Such a flange is a generally flat plate with a central hole and a bolt circle having a plurality of bolt holes extending circumferentially around the flange. Such a separately manufactured flange is then adhesively bonded around the end of a section of pipe. Two such flanges may then be bolted together to interconnect pipe sections.
Although nominally a flat plate, a central hub may be formed around the hole through the flange to enlarge the area for adhesive bonding to the pipe. Various grooves, seal rings and the like may be provided on the mating faces of such flanges for sealing the joint. Such geometric details of the pipe flange are not material to practice of this invention which relates to the fiber reinforcement of the flange.
It has been the practice to form such pipe flanges by winding resin impregnated rovings of glass fibers in a circumferential "slot" of a mandrel. A plurality of such rovings are gradually built up by rotating the mandrel until the slot is filled to the desired depth. After the plastic resin is cured, the flange is stripped from the mandrel for final machining. This machining typically involves turning the circumference, primarily for appearance and drilling a plurality of bolt holes in circle between the central hole and the circumference of the flange. It is also feasible to machine sealing rings or grooves on the face of the flange.
Sometimes a problem is encountered with flanges made by wrapping rovings of fiber around a mandrel. When the holes are drilled for the bolt circle, superficial cracks may appear in the bolt holes or on the surfaces of the flange between the bolt holes, probably due to shrinkage stresses built up from curing of the resin. This may also occur when bolts are over torqued in the holes. Testing has shown that such cracks do not adversely affect the flange's performance, however, customers unfamiliar with this information object to the cracks and may reject parts showing them. Thus, although the cracks are essentially "cosmetic" and may not harm the product in service, they may not be acceptable to prospective customers.
It is, therefore, desirable to provide pipe flanges with appropriate reinforcement for preventing the formation of such cracks.